208 PHILOSOPHY OF STORMS. 



force, which will cause the wind to set in at the beginning 

 of these storms, not exactly towards the centre of them, but 

 towards a point west of that centre, and if this single cir- 

 cumstance should be observed without attending to all the 

 phenomena, it would undoubtedly give rise to a suspicion, 

 that the wind in these storms rotates from right to left. 

 And if to this circumstance be added, that these storms are 

 nearly round in this latitude, and that the air at some mod- 

 erate distance around them is nearly calm, the investigator 

 will be confirmed in his first impression, and perhaps not 

 even think it worth while to mark on his chart, by arrows, 

 the course of the wind in the simultaneous observations at 

 his command. And if to all this is added his belief, that 

 the air in a cloud is denser and heavier than surrounding 

 air at the same elevation, he will consider that it amounts 

 to absolute demonstration, that there must be a whirl, as 

 that is the only possible means of causing a depression of 

 the barometer under all these circumstances, in the middle 

 of the storm. Again, if he believes that the cold air from 

 above, coming down in the centre of the whirlwind, which 

 it would do, mingles with the warm air below, and thus 

 produces condensation of its vapor, he thinks he has got 

 hold of a fact, which enables him to explain many phe- 

 nomena connected with the storm, though the whirlwind 

 itself remain^ unexplained, as it always must. But if he 

 will examine this subject a little more minutely, he will 

 find, that if air should descend from a height sufficiently 

 great to double its density at the surface of the earth, its 

 dew point will be raised only 20, and its temperature by 

 increased pressure about 90, and that it will then be ex- 

 tremely dry, even if it had been saturated at the commence- 

 ment of its descent. In fact, it would then be able to con- 

 tain about eight times as much vapor as it contained before 

 its descent; for at these low temperatures every increase of 

 temperature of 20 doubles its power of containing vapor, 



